In conventionally proposed power output apparatuses, a generator and a motor are connected to respective rotational elements of a planetary gear mechanism linked to a crankshaft of an engine, while a battery is provided to allow transmission of electric power to and from the generator and the motor. For example, a prior art power output apparatus disclosed in Japanese Patent Laid-Open Gazette No. 2001-304032 mainly uses the output power of the motor and consumes the output power of the engine in the event of an insufficiency of the power output from the motor in an ordinary state without degradation detection of a catalyst included in an emission control device provided in an exhaust system of the engine. This power output apparatus varies the output power of the motor while keeping the output power level of the engine substantially constant during degradation detection of the catalyst. Such control intends to attain the quick detection of degradation of the catalyst.